Structural, Geomorphic, and Depositional Characteristics of Contiguous and Broken Foreland Basins: Examples from the Eastern Flanks of the Central Andes in Bolivia and NW Argentina
Abstract:In this chapter, we contrast the elements of a typical, contiguous foreland basin system in the Bolivian Andes with the broken foreland farther south in northwestern Argentina. We illustrate differences in deposition and geomorphic shape that arise from the structural conditions to which these two systems are subjected. Generally, the principal elements of foreland-basin systems result mainly from accommodation space created by the flexural response of the crust to the topographic load of a fold-and-thrust bel… Show more
“…What makes the Dunsgam Chu section special along the Himalayan arc is its location north of the Shillong Plateau, a unique basementâcored uplift in the Himalayan foreland (Figure ). At the longitude of central and eastern Bhutan, the Himalayan foreland corresponds to a broken foreland basin [e.g., Jordan , ; Strecker et al ., ], rather than a classical foreland system [e.g., DeCelles and Giles , ]. In this area, the foredeep is very shallow (maybe <1 km), in contrast to areas west and east of Bhutan (depths >4 km) [ Dasgupta , ; Verma and Mukhopadhyay , ; Vernant et al ., , Figure 1b].…”
The Shillong Plateau is a unique basementâcored uplift in the foreland of the eastern Himalaya that accommodates part of the IndiaâEurasia convergence since the late Miocene. It was uplifted in the late Pliocene to 1600âm, potentially inducing regional climatic perturbations by orographically condensing part of the Indian Summer Monsoon (ISM) precipitations along its southern flank. As such, the eastern HimalayaâShillong PlateauâISM is suited to investigate effects of tectonics, climate, and erosion in a mountain rangeâbroken foreland system. This study focuses on a 2200âm thick sedimentary section of the Siwalik Group strategically located in the lee of the Shillong Plateau along the Dungsam Chu at the front of the eastern Bhutan Himalaya. We have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating, combined with sedimentological and palynological analyses. We show that (1) the section was deposited between ~7 and 1âMa in a marginal marine deltaic transitioning into continental environment after 5âMa, (2) depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the Shillong Plateau had an unexpected limited impact on the paleoclimate of the Bhutanese foothills, and (3) the diminution of the flexural subsidence in the basin and/or of the detrital input from the range is attributable to a slowdown of the displacement rates along the Main Boundary Thrust in eastern Bhutan during the latest MioceneâPleistocene, in response to increasing partitioning of the IndiaâEurasia convergence into the active faults bounding the Shillong Plateau.
“…What makes the Dunsgam Chu section special along the Himalayan arc is its location north of the Shillong Plateau, a unique basementâcored uplift in the Himalayan foreland (Figure ). At the longitude of central and eastern Bhutan, the Himalayan foreland corresponds to a broken foreland basin [e.g., Jordan , ; Strecker et al ., ], rather than a classical foreland system [e.g., DeCelles and Giles , ]. In this area, the foredeep is very shallow (maybe <1 km), in contrast to areas west and east of Bhutan (depths >4 km) [ Dasgupta , ; Verma and Mukhopadhyay , ; Vernant et al ., , Figure 1b].…”
The Shillong Plateau is a unique basementâcored uplift in the foreland of the eastern Himalaya that accommodates part of the IndiaâEurasia convergence since the late Miocene. It was uplifted in the late Pliocene to 1600âm, potentially inducing regional climatic perturbations by orographically condensing part of the Indian Summer Monsoon (ISM) precipitations along its southern flank. As such, the eastern HimalayaâShillong PlateauâISM is suited to investigate effects of tectonics, climate, and erosion in a mountain rangeâbroken foreland system. This study focuses on a 2200âm thick sedimentary section of the Siwalik Group strategically located in the lee of the Shillong Plateau along the Dungsam Chu at the front of the eastern Bhutan Himalaya. We have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating, combined with sedimentological and palynological analyses. We show that (1) the section was deposited between ~7 and 1âMa in a marginal marine deltaic transitioning into continental environment after 5âMa, (2) depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the Shillong Plateau had an unexpected limited impact on the paleoclimate of the Bhutanese foothills, and (3) the diminution of the flexural subsidence in the basin and/or of the detrital input from the range is attributable to a slowdown of the displacement rates along the Main Boundary Thrust in eastern Bhutan during the latest MioceneâPleistocene, in response to increasing partitioning of the IndiaâEurasia convergence into the active faults bounding the Shillong Plateau.
“…Synorogenic sedimentary basins are preserved on both orogenic flanks, including forearc basins controlled by diverse structures and retroarc hinterland and foreland basins mostly associated with shorteningâinduced topographic loading and lithospheric flexure (Horton, , , ; Horton & DeCelles, ; Jordan, ; Jordan et al, ; Watts et al, ). Structural, stratigraphic, and thermochronologic results show that the locus of Late CretaceousâCenozoic shortening has advanced eastward toward the South American craton (Carrapa et al, ; Carrapa & DeCelles, ; DeCelles & Horton, ; Gubbels et al, ; Elger et al, ; Ege et al, ; Horton et al, ; McQuarrie et al, , ; Strecker et al, ).…”
Construction of the Andes has been governed largely by fluctuating contractional, neutral, and extensional tectonic regimes during differing degrees of mechanical coupling along the convergent plate boundary. These three contrasting regimes are likely regulated by geodynamic variations in absolute trenchward motion of South America and episodic regional shifts in the geometry of the subducting oceanic slab. Alternations among these three modes are revealed in syntheses of MesozoicâCenozoic crustal deformation, basin evolution, and arc magmatism along orogenâperpendicular transects across the central and southern Andes at 23°S, 35°S, and 43°S. Despite considerable alongâstrike dissimilarities in the magnitude of deformation, a comparable tectonic regime typified the early Andean history, as defined by a transition from Late JurassicâEarly Cretaceous postrift thermal subsidence to Late Cretaceous retroarc shortening. A key contradiction is expressed for the late middle Eocene to earliest Miocene, when neutral to extensional conditions affected retroarc to forearc regions, except in parts of the central Andes, which experienced retroarc shortening with only localized forearc extension. This mixedâmode scenario during Paleogene time may reflect decoupling along the plate boundary (possibly during slab rollback within a retreating subduction system) in which widespread stasis or lowâmagnitude extension dominated the southern Andes but was inhibited in the strongly shortened central Andes at 15â25°S where shallow subduction and/or high topography boosted plate coupling. Comparable temporal and spatial shifts in tectonic regime along the Andes and other convergent margins can be related to variable degrees of coupling during firstâorder plateâscale changes in convergence and secondâorder regional cycles of slab shallowing and steepening.
“…Foreland lithosphere is sensitive to a wide variety of processes directly to indirectly related to orogenesis: lithospheric flexure, where the continental plate is forced to bend under the weight of the orogen (creating a foreland basin system), lithospheric fragmentation, where the plate breaks in response to orogenic compression (giving place to a broken foreland basin system) (DeCelles and Giles, 1996;Strecker et al, 2011) and dynamic subsidence and uplift (e.g. Braun et al, 2010).…”
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